The mail only account is just that, mail only. You can not use the address book or any other features that are provided to a full membership account, including aliases.
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Oops, sorry, my wife just informed me, the one with the mail only account attached to my full account, that she does have an address book... now she is pestering me as to why it is not synced sigh.. it appears that you can not sync between them, wonder if there is a way to have the iSync app sync with two .mac accounts.